1. Field of the Invention
The present invention relates to an apparatus and method for quenching, and to an on-stream catalyst replacement during hydroprocessing of a hydrocarbon feed stream.
More particularly, the present invention provides for an apparatus and method for quenching in hydroprocessing of a hydrocarbon feed stream through a hydroprocessing vessel and for economically utilizing space within the hydroprocessing vessel over a wide range of processing rates without substantial fluidization or ebullation of a packed bed of catalyst during high counterflow rates of the hydrocarbon feed and a hydrogen containing gas through the packed bed, while maintaining continuous or intermittent replacement of catalyst for plug-like flow of the bed through the vessel. Such plug flow with high processing rates is obtained by selecting the size, shape and density of the catalyst particles to prevent ebullation and bed expansion at the design flow rate so as to maximize the amount of catalyst in the vessel during normal operation and during catalyst transfer. Catalysts are of uniform or substantially the same size, shape and density and are selected by measuring bed expansion, such as in a large pilot plant run, with hydrocarbon, hydrogen and catalyst at the design pressures and flow velocities within the available reaction volume of the vessel. Catalyst is removed from the bed by laminar flow of the catalyst particles in a liquid slurry system in which the liquid flow line is uniform in diameter, and substantially larger than the catalyst particles, throughout the flow path between the reactor vessel and a pressurizable vessel including passageways through the flow control valves.
2. Description of the Prior Art
Hydroprocessing or hydrotreatment to remove undesirable components from hydrocarbon feed streams is a well known method of catalytically treating such heavy hydrocarbons to increase their commercial value. "Heavy" hydrocarbon liquid streams, and particularly reduced crude oils, petroleum residua, tar sand bitumen, shale oil or liquified coal or reclaimed oil, generally contain product contaminants, such as sulfur, and/or nitrogen, metals and organo-metallic compounds which tend to deactivate catalyst particles during contact by the feed stream and hydrogen under hydroprocessing conditions. Such hydroprocessing conditions are normally in the range of 212 degree(s) F to 1200 degree(s) F (100 degree(s) to 650 degree(s) C.) at pressures of from 20 to 300 atmospheres. Generally such hydroprocessing is in the presence of catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other metallic element particles of alumina, silica, magnesia and so forth having a high surface to volume ratio. More specifically, catalyst utilized for hydrodemetallation, hydrodesulfurization, hydrodenitrification, hydrocracking etc., of heavy oils and the like are generally made up of a carrier or base material; such as alumina, silica, silica-alumina, or possibly, crystalline aluminosilicate, with one more promoter(s) or catalytically active metal(s) (or compound(s)) plus trace materials. Typical catalytically active metals utilized are cobalt, molybdenum, nickel and tungsten; however, other metals or compounds could be selected dependent on the application.
Because these reactions must be carried out by contact of a hydrogen-containing gas with the hydrocarbon feed stream at elevated temperatures and pressures, the major costs of such processing are essentially investment in vessels and associated furnaces, heat exchangers, pumps, piping and valves capable of such service and the replacement cost of catalyst contaminated in such service. Commercial hydroprocessing of relatively low cost feed stocks such as reduced crude oils containing pollutant compounds, requires a flow rate on the order of a few thousand up to one hundred thousand barrels per day, with concurrent flow of hydrogen at up to 10,000 standard cubic feet per barrel of the liquid feed. Vessels capable of containing such a reaction process are accordingly cost-intensive both due to the need to contain and withstand corrosion and metal embrittlement by the hydrogen and sulfur compounds, while carrying out the desired reactions, such as demetallation, denitrification, desulfurization, and cracking at elevated pressure and temperatures. For example, because of metallurgy and safety requirements, such vessels may cost on the order of $700.00 per cubic foot of catalyst capacity. Thus a vessel capable of handling 25,000 barrels per day of a hydrocarbon feed stream may run on the order of $4,000,000 to $5,000,000. Pumps, piping and valves for handling fluid streams containing hydrogen at such pressures and temperatures are also costly, because at such pressures seals must remain hydrogen impervious over extended service periods of many months.
Further, hydroprocessing catalyst for such a reactor, which typically contains metals such as titanium, cobalt, nickel, tungsten, molybdenum, etc., may involve a catalyst inventory of 500,000 pounds or more at a cost of $2 to $4/lb. Accordingly, for economic feasibility in commercial operations, the process must handle high flow rates and the vessel should be filled with as much catalyst inventory as possible to maximize catalyst activity and run length. Additionally, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams of varying amounts of contaminants such as sulfur, nitrogen, metals and/or organic-metallic compounds, such as those found in a wide variety of the more plentiful (and hence cheaper) reduced crude oils, residua, or liquified coal, tar sand bitumen or shale oils, as well as used oils, and the like. The following three acceptable reactor technologies are currently available to the industry for hydrogen upgrading of "heavy" hydrocarbon liquid streams: (i) fixed bed reactor systems; (ii) ebullated or expanded type reactor systems which are capable of onstream catalyst replacement and are presently known to industry under the trademarks H-Oil.sub.R and LC-Fining.sub.R ; and (iii) the substantially packed-bed type reactor system having an onstream catalyst replacement system, as more particularly described in U.S. Pat. No. 5,076,908 to Stangeland et al, having a common assignee with the current inventions and discoveries.
A fixed bed reactor system may be defined as a reactor system having one or more reaction zone(s) of stationary catalyst, through which feed streams of liquid hydrocarbon and hydrogen flow downwardly and concurrently with respect to each other.
An ebullated or expanded bed reactor system may be defined as a reactor system having an upflow type single reaction zone reactor containing catalyst in random motion in an expanded catalytic bed state, typically expanded from 10% by volume to about 35% or more by volume above a "slumped" catalyst bed condition (e.g. a non-expanded or non-ebullated state).
As particularly described in U.S. Pat. No. 5,076,908 to Stangeland et al, the substantially packed-bed type reactor system is an upflow type reactor system including multiple reaction zones of packed catalyst particles having little or no movement during normal operating conditions of no catalyst addition or withdrawal. In the substantially packed-bed type reactor system of Stangeland et al., when catalyst is withdrawn from the reactor during normal catalyst replacement, the catalyst flows in a downwardly direction under essentially plug flow or in an essentially plug flow fashion, with a minimum of mixing with catalyst in layers which are adjacent either above or below the catalyst layer under observation.
Of the three acceptable reactor systems, most hydroconversion reactor systems presently in operation on a worldwide basis are fixed bed reactors wherein a liquid hydrocarbon feed and a hydrogen stream flow concurrently through the catalyst beds in a downward flow path. While these fixed bed downflow type processes assure maximum density or volume of catalyst within a reaction zone without expansion of the bed, they are limited by the tendency of the catalyst to form local deposits of feed metals and other contaminates, particularly in the top catalyst bed (i.e. first reaction zone), affecting distribution and reaction rates. As reactor average temperatures are progressively increased to maintain processing objectives under conditions of increasing local metal deposits, catalyst deactivation due to carbon deposition accelerates. When processing objectives can no longer be maintained due to catalyst deactivation (i.e. normally recognized as "End of Run" conditions), the reactor system must be taken offstream for catalyst regeneration or replacement. Accordingly, in general, it is preferred to counterflow the catalyst and process fluid streams relative to each other. However, as noted above, when the process feed rates are high, the volume of catalyst that can be contained by the vessel may be as little as 10% of the original settled volume. At lower fluid velocities, catalyst volume may be up to about 80% to 90%, but useful reaction space for the process is still wasted and turbulence causes axial mixing of the catalyst. Therefore, one particular object of the present invention is to run a counterflow processing system where the catalyst and fluid velocity combinations limit bed expansion to less than 10% by length (more preferably less than about 5% by length, most preferably less than 2% or even less than 1% by length) beyond a substantially full axial length of the bed in a packed bed state.
It is also known to use a series of individual vessels stacked one above the other, with fluid flow either cocurrent or counterflow to catalyst. In such a process, catalyst moves by gravity from the upper vessel to a lower vessel by periodically shutting off, or closing, valves between the individual vessels. In a counterflow system, this permits removal of catalyst from the lowermost or first stage vessel, where the most contaminated, or raw, feed stock, originally contacts the catalyst. In this way, most of the major contaminating components in the hydrocarbon stream are removed before the hydrocarbon material reaches major conversion steps of the process performed in higher vessels of the stacked series. Thus, most of the deactivating components of the feed stream are removed before it reaches the least contaminated catalyst added to the topmost vessel. However, such systems require valves suitable for closing off catalyst flow against catalyst trapped in the line. Hence, valve life is relatively short and down-time for replacement or repair of the valves is relatively costly.
Since the late 1960's, there have been several heavy oil hydroprocessing units built and brought on stream that utilize the ebullated or expanded catalyst bed reactor technology where a hydrocarbon feed stream and hydrogen gas flow upwardly through a dilute phase reaction zone of catalyst in random motion. Stated alternatively, continuous operation of an ebullated or expanded bed hydroprocessing system include the upward flow of a hydrocarbon feed stream and hydrogen gas through a single catalyst containing vessel or a series of catalyst containing vessels. Reactor liquid is recirculated internally at rates sufficient to expand or ebullate the catalyst to produce a dilute phase reaction zone of catalyst in random or ebullating motion. Catalyst is replaced by continuous or periodic, onstream removal of catalyst from the vessel followed by addition. As noted above, such ebullation tends to increase the fluid volume in the vessel relative to catalyst volume necessary to hydroprocess the feed stream and hydrogen with the catalyst, with adequate contact time to react the fluids. Further, such ebullated beds tend to result in separation or segregation of "fines" from the larger (and heavier) particles as they pass downwardly through the upflow streams. As frequently happens, and especially where the catalyst is locally agitated, as by eddy currents, the particles tend to abrade by such higher flow rates of the feed streams through the ebullating bed. Depending on the size of the fines, they either travel upward where they contaminate the product or they tend to accumulate in the reactor because they cannot work their way down to the bottom of the bed. Such counterflow systems have also been used because of the relative ease of withdrawing limited amounts of the ebullated catalyst in a portion of the reacting hydrocarbon and hydrogen fluids, particularly where such turbulent flow of the catalyst is needed to assist gravity drainage through a funnel-shaped opening into a central pipe at the bottom of a vessel.
While it has been proposed heretofore to use plug-flow or packed-bed flow of catalyst to reduce such agitation and thus assure uniform disbursement of hydrogen throughout the liquid volume flowing upwardly through the catalyst bed, in general such flow has been controlled by limiting the maximum flow rate that can be tolerated without ebullating or levitating the bed more than about 10%. Further in prior systems where expansion of the bed is limited, hydrogen flow rates are made sufficiently high at the bottom of the bed to assure relative turbulence of the catalyst at the withdrawal point in the vessel. While this does assure such turbulence, it also wastes space, damages the catalyst and permits direct entrainment of hydrogen with catalyst entering the withdrawal tube. Such turbulent flow of catalyst is apparently necessary to assist gravity removal of catalyst from the vessel.
The basic process design of the ebullated bed reactors with appropriate mechanical features overcome some of the limitations of the conventional fixed bed reactor. The ebullated or expanded catalyst bed reactor schemes provide ability to replace catalyst on stream and operate with a very "flat" reaction zone temperature profile instead of the steeper pyramiding profile of conventional fixed bed reactors. The nature of the process, with a broad spectrum of catalyst size, shape, particle density, and activity level in random motion in a "dilute phase reaction zone," creates near isothermal temperature conditions, with only a few degrees temperature rise from the bottom to the top of the reaction zone. Quench fluids are not normally required to limit reaction rates except in cases when series reactors are applied. In other words, the reactor internal recycle oil flow, used to expand (or ebullate) the catalyst bed and maintain distribution (typically 10 to 1 ratio of fresh oil feed) serves also as "internal quench" to control reaction rate and peak operating temperatures. Because the highest temperatures experienced in the reactors are only a few degrees above the average temperature required to maintain processing objectives and not the higher "end of run" peak temperatures experienced in fixed bed reactor systems, the accelerated fouling rate of the catalyst by carbon deposition experienced in conventional fixed bed reactor systems at "end of run" conditions is minimized; however, the normal carbon deposition rate is much greater than that of the fixed bed reactor due to overall operating conditions.
Unfortunately, implementing the ebullated bed technology results in inefficient use of reactor volume and less than optimum usage of hydroconversion catalyst. Catalyst replacement rates for ebullated bed reactors are based on maintaining "catalyst equilibrium conditions" necessary to maintain processing objectives. The backmixing nature of ebullated catalyst beds, combined with the characteristics of the typical extrudate catalyst particulate used (i.e. a full range of sizes and shapes), promote isothermal temperature conditions but create selectivity difficulties in regard to the withdrawal of expended catalyst. Fresh or partially expended catalyst commingle with expended catalyst withdrawn from the bottom of the catalyst bed requires complicated procedures and equipment to recover, and are usually discarded with minimum recovery value. In other words, use of various size and shape catalyst in ebullated bed type reactors leads to somewhat inefficient use of catalyst value.
The additional reactor volume required for the ebullated bed process is to accommodate the expansion of the catalyst load by 25-35% of its original slumped (or "packed bed") volume or height, by controlling the velocity of an internal liquid recycle stream. Space required within an ebullated bed reactor for the disengagement of solids and catalyst bed level controls, and the space required to satisfy suction conditions for the reactor internal recycle pump, combined with the space the pump suction line occupies, consumes a substantial amount of space available within the ebullated bed reactor. Additional disadvantages of the ebullated bed technology are the added cost, maintenance, and the reliability of a single supply source for the reactor recycle pump which is required to expand the catalyst bed. In order to compare efficient use of reactor volume purchased, the following examples are offered.
If ebullating bed reactor technology is implemented and 13-foot diameter reactors are selected, the tangent line to tangent line dimensions required for the 13-foot diameter reactors would be approximately 60 feet in order to load approximately 5,000 cubic feet (or about 175,000 lb) of typical hydroprocessing catalyst. Thus, the 5,000 cubic feet of catalyst occupies about 63% by volume of the approximately 7,900 cubic feet of reactor volume available between the bottom and top tangent line of the reactor. In the case of fixed bed reactors, in order to load 5,000 cubic feet of typical hydroprocessing catalyst in 13-foot diameter reactors would require tangent line to tangent line dimensions for the 13-foot diameter reactors of about 43 feet; however, the operating run length for the fixed bed reactors would be short as catalyst could not be replaced on stream. Should the 60-foot tangent line to tangent line dimensions required for the ebullated bed reactors be maintained for a fixed bed reactor, an additional catalyst volume of approximately 2000 cubic feet could be loaded. In order to load 5,000 cubic feet of typical hydroprocessing catalyst in a 13-foot diameter bed reactor with the broad features and descriptions as disclosed in U.S. Pat. No. 5,076,908 to Stangeland et al, would require tangent line to tangent line dimensions of approximately 41 feet. There would be a reduction of reactor empty weight of between 100 to 200 tons, depending on the design pressure specification. Should the 60-foot tangent line to tangent line dimensions for a Stangeland et al reactor be maintained, an additional catalyst volume of approximately 2500 cubic feet could be loaded.
As particularly distinguished from prior known methods of on-stream catalyst replacement in hydroprocessing, the method and apparatus in U.S. Pat. No. 5,076,908 to Stangeland et al more specifically provides a system wherein plug flow of the catalyst bed is maintained over a wide range of counterflow rates of a hydrocarbon feed stream and hydrogen gas throughout the volume of the substantially packed catalyst bed. Such packed bed flow maintains substantially maximum volume and density of catalyst within a given vessel's design volume by controlling the size, shape and density of the catalyst so that the bed is not substantially expanded at the design rate of fluid flow therethrough. The proper size, shape and density are determined by applying coefficients gained during extensive studying of bed expansion in a large pilot plant runs with hydrocarbon, hydrogen and catalyst at the design pressures and flow velocities as particularly described below.
To further control such packed bed flow, the bed level of catalyst within the vessel is continuously measured, as by gamma ray absorption, to assure that little ebullation of the bed is occurring. Such control is further promoted by evenly distributing both the hydrogen and liquid feed throughout the length of the bed by concentrically distributing both the hydrogen gas component and the hydrocarbon fluid feed component in alternate, concentric annular paths across the full horizontal cross-sectional area of the vessel as they both enter the catalyst bed. Additionally, and as desirable, hydrogen is evenly redistributed and if needed, augmented, through a quench system at one or more intermediate levels along the length of the catalyst bed. Equalizing hydrogen and liquid feed across the full horizontal area along the length of the packed particle bed prevents local turbulence and undesirable vertical segregation of lighter particles from heavier particles flowing in a plug-like manner downwardly through the vessel.
Further in accordance with the method that is more particularly disclosed and described in U.S. Pat. No. 5,076,908 to Stangeland et al, a system for replacing catalyst during continuing operation of the non-ebullating bed is assisted by carrying out the process at relatively high liquid feed rates, even without ebullation of the bed. In a preferred form, the catalyst transfer system includes an inverted J-tube as the withdrawal tube, so that the tube opens downwardly adjacent the center of the lower end of the vessel and directly above a center portion of the surrounding annular flow paths of liquid and gas into the catalyst bed. Thus, the intake for catalyst is out of the direct flow of such streams, and particularly the gas flow. In such a preferred form the annular flow paths are through a conical or pyramidal screen, or perforated plate, which supports the bed or column of catalyst across the vessel through a plurality of radially spaced apart and axially elongated concentric rings, or polygons, supported by radial arms extending from the center of the vessel to the cylindrical side wall of the vessel. Each ring is formed by a pair of peripheral members extending between the radial arms directly under the conical screen so that this forms a circular gas pocket at the upper level in each ring so that between each pair of said peripheral members alternate rings of gas and hydrocarbon liquid enter the bed simultaneously.
In accordance with a further preferred form of the method and apparatus that is more particularly disclosed and described in Stangeland et al, catalyst is both withdrawn from the bed and added to the vessel under laminar flow conditions as a liquid slurry to avoid abrasion and size segregation of particles during such transfer. Both the supply and withdrawal flow lines have a minimum diameter of at least five times and, preferably more than twenty times, the average diameter of the catalyst particles. Further, the flow lines are of uniform diameter throughout their length from either the catalyst supply chamber to the vessel, or from the vessel to the receiving chamber, including the through bore of a rotatable ball of the isolating, pressure control valves, known commercially as "full-port valves". Additionally, in each case a flush line is connected to the flow line between the isolating valve and the reactor vessel so that liquid hydrocarbon may be used to flush the line of catalyst or catalyst fines if necessary, before the valve ball is closed. Preferably, but not necessarily, the withdrawal line may include means for flowing auxiliary hydrogen back into the reactor through the withdrawal tube to prevent coking due to hydrogen starvation near or in the withdrawal tube.
The prior art does not disclose or suggest the above enumerated and pertinent features of either the total system or significant portions of such a system in U.S. Pat. No. 5,076,908 to Stangeland et al, as disclosed by the following patents:
Jacquin et al. U.S. Pat. No. 4,312,741, is directed toward a method of on-stream catalyst replacement in a hydroprocessing system by controlling the feed of hydrogen gas at one or more levels. Catalyst, as an ebullated bed counterflows through the reactor but is slowed at each of several levels by horizontally constricted areas which increase the hydrogen and hydrocarbon flow rates to sufficiently locally slow downward flow of catalyst. While local recycling thus occurs at each such stage, rapid through-flow of fresh catalyst, with resultant mixing with deactivated or contaminated catalyst, is suppressed. The ebullating bed aids simple gravity withdrawal of catalyst from the vessel. Improvement of the disclosed system over multiple vessels with valves between stages is suggested to avoid the risk of rapid wear and deterioration of valve seals by catalyst abrasion.
Kodera et al. U.S. Pat. No. 3,716,478, discloses low linear velocity of a mixed feed of liquid and H.sub.2 gas to avoid expansion (or contraction) of catalyst bed. By low linear velocity of fluid upflow, gas bubbles are controlled by flow through the packed bed, but the bed is fluidized by forming the bottom with a small cross-sectional area adjacent the withdrawal tube. This assists discharge of catalyst without backmixing of contaminated catalyst with fresh catalyst at the top of the single vessel. The range of the bed level in the vessel is from 0.9 to 1.1 of the allowable bed volume (.+-.10%) due to fluid flow through the bed. A particular limitation of the system is that flow of the fluids undergoing catalytic reaction is restricted to a rate that will not exceed such limits, but must be adequate to ebullate the bed adjacent the catalyst withdrawal tube. Alternatively, injection of auxiliary fluid from a slidable pipe section is required. The patentees particularly specify that the diameter of the lower end of the vessel is smaller to increase turbulence and ebullation of catalyst adjacent the inlet to the catalyst withdrawal line. Fluidization of catalyst is accordingly indicated to be essential to the process. However the disclosed gas flow rates are well below commercial flow rates and there is no suggestion of temperatures or pressures used in the tests or the size, density or shape of the catalyst.
Bischoff et al, U.S. Pat. No. 4,571,326, is directed to apparatus for withdrawing catalyst through the center of a catalyst bed counterflowing to a liquid hydrocarbon and gas feed stream. The system is particularly directed to arrangements for assuring uniform distribution of hydrogen gas with the liquid feed across the cross-sectional area of the bed. Such uniform distribution appears to be created because the bed is ebullating under the disclosed conditions of flow. Accordingly, considerable reactor space is used to initially mix the gas and hydrocarbon liquid feeds in the lower end of the vessel before flowing to other bottom feed distributors. The feeds are further mixed at a higher level by such distributor means in the form of "Sulzer Plates" or a "honeycomb" of hexagonal tubes beneath a truncated, conical, or pyramidal-shaped funnel screen. The arrangement may include an open ramp area parallel to the underside of the screen between the tube or plate ends. Further, to maintain gas distribution along the length of the catalyst bed, quench gas is supplied through upflowing jets in star-shaped or annular headers extending across middle portions of the vessel. The arrangement for withdrawal of spent catalyst requires ebullation of at least the lower portion of the bed. As noted above, added vessel space for uniform mixing of hydrogen and feed before introducing the fluids into an ebullated bed, as well as an ebullating bed, increases the required size of the hydroprocessing vessel, increases catalyst attrition, increases catalyst bed mixing and substantially increases initial, and continuing operating costs of the system.
Bischoff et al. U.S. Pat. No. 4,639,354, more fully describes a method of hydroprocessing, similar to U.S. Pat. No. 4,571,326 wherein similar apparatus obtains uniform ebullation through the vertical height of a catalyst bed, including a quench gas step.
Meaux U.S. Pat. No. 3,336,217, is particularly directed to a catalyst withdrawal method from an ebullating bed reactor. In the system, catalyst accumulating at the bottom of a vessel and supported on a flat bubble-tray may be withdrawn through an inverted J-tube having a particular ratio of the volume of the short leg of the J-tube to the longer leg.
The diameter of the J-tube is suited only to flow of catalyst from a body of catalyst ebullated by the upflowing hydrocarbon feed and gas.
U.S. Pat. Nos. 4,444,653 and 4,392,943, both to Euzen, et al., disclose removal systems for catalyst replacement in an ebullating bed. In these patents, the fluid charge including hydrocarbon containing gas is introduced by various arrangements of downwardly directed jets acting laterally against or directly onto the conical upper surface of the bed support screen or screens. Alternatively, the feed is introduced through a conical screen after passing through a distributor arrangement of tortuous paths or a multiplicity of separate tubes to mix the gas and liquid feed over the conical screen. Such arrangements use a considerable volume of the pressure vessel to assure such mixing.
U.S. Pat. Nos. 3,730,880 and 3,880,569, both to Van der Toorn, et al., disclose a series of catalytic reactors wherein catalyst moves downwardly by gravity from vessel to vessel through check valves. As noted above, such valves require opening and closing to regulate the rate of flow, or to start and stop catalyst transfer, with catalyst in the valve flow path. Feed of process fluids is either co-current or countercurrent through the catalyst bed.
Van ZijllLanghaut et al. U.S. Pat. No. 4,259,294, is directed to a system for on-stream catalyst replacement by entrainment of the catalyst in oil pumped as a slurry either to withdraw catalyst from or to supply fresh catalyst to, a reactor vessel. Reacting feed is suggested to be either co-current or countercurrent with catalyst flow through the reactor. Valves capable of closing with catalyst in the line, or after back-flow of slurry oil, are required to seal off the catalyst containing vessel at operating temperatures and pressures from the receiving reactor vessel, or isolate the catalyst receiving lock hopper from the withdrawal section of the vessel.
Carson U.S. Pat. No. 3,470,090, and Sikama U.S. Pat. No. 4,167,474, respectively illustrate multiple single bed reactors and multi-bed reactors in which catalyst is replaced either continuously or periodically. The feed and catalyst flow co-currently and/or radially. Catalyst is regenerated and returned to the reactor, or disposed of. No catalyst withdrawal system is disclosed apart from either the configuration of the internal bed support or the shape of the vessel bottom to assist gravity discharge of catalyst.
One of the basic principles and teachings of Stangeland et al in U.S. Pat. No. 5,076,908, is that by specifically selecting the size, shape, and density of the catalyst pellets, combined with appropriate control of process liquid and gas velocities, random motion and backmixing of the catalyst can be minimized, and plugflow characteristics of the catalyst downward and the liquid and gas flows upward, maximized. Stangeland et al economically utilizes space within a hydroprocessing vessel over a wide range of processing rates without substantial random motion or ebullation of a packed bed of catalyst during high counterflow rates of the hydrocarbon feed and a hydrogen containing gas through the packed bed, while maintaining continuous or intermittent replacement of catalyst for plug-like flow of the bed through the vessel. Such plug flow with high processing rates is obtained by Stangeland et al by selecting the size, shape and density of the catalyst particles to prevent ebullation and bed expansion at the design flow rate so as to maximize the amount of catalyst in the vessel during normal operation and during catalyst transfer. Catalysts are selected utilizing data gained while studying catalyst bed expansion, such as in a large pilot plant run, with hydrocarbon, hydrogen and catalyst at the design pressures and flow velocities within the available reaction volume of the vessel. Catalyst is removed from the bed by Stangeland et al through laminar flow of the catalyst particles in a liquid slurry system in which the liquid flow line is uniform in diameter, and substantially larger than the catalyst particles, throughout the flow path between the reactor vessel and a pressurizable vessel including passageways through the flow control valves.
Experience in practicing the principles and teachings of Stangeland et al, U.S. Pat. No. 5,076,908 has demonstrated maximum utilization of available reactor volume, operability of reactors with feed upflowing through dense phase "packed catalyst beds," and the ability to replace catalyst on stream. Catalyst selection by size, shape and density of catalyst particles to fit each unique processing case has been validated as a key element in optimizing performance of an upflow type hydroprocessing reactor, regardless if "packed bed" or "expanded bed" reaction zones are involved.
In other words, through experience with same or uniform size, shape and density catalyst, additional technical advantages and opportunities have been discovered to further enhance performance and maximize catalyst life--not obtainable when random particle size catalyst is used in units where catalyst is replaced on stream. Therefore, a particular object of the present invention is to extend the life of hydroprocessing catalyst from a hydroconversion reaction zone. Experience in practicing the principles and teachings of Stangeland et al has also demonstrated that when operating on once through catalyst replacement mode, the temperature profile in the final reaction zone (i.e. the freshest most active catalyst) could affect catalyst fouling rate by carbon deposition. The present invention provides for a method to adjust catalyst activity profile (i.e. temperature profile) in order to more favorably enhance overall hydroconversion reactor performance, thus optimizing reactor performance and extending the life of the hydroprocessing catalyst in the hydroconversion reactor.
It is well known to those skilled in the art of hydrogen upgrading of heavy hydrocarbon streams in certain types of bed reactor systems that interstage quenching is required between multi-catalyst bed reaction zones for temperature control. Typically, the temperature of a first reaction zone in multi-catalyst bed reaction zones is controlled by the feed heater outlet/reactor inlet temperature, and the temperature of each succeeding reaction zone is controlled by an injection of a quench medium at the exit of a preceding zone.
Quench gas has historically been the cooling medium of choice for most fixed bed or packed bed reactor systems that are presently operating onstream or are in preliminary engineering stages. The genesis of quench gas for utilization as a cooling medium in a hydroprocessing system is normally from a discharge of a hydroprocessing system's recycle hydrogen compressor, except in a few special applications where fresh high-purity make-up hydrogen may be selected. While quenching with gas performs the necessary function of reactor temperature profile control and also serves to replenish hydrogen chemically consumed by the process, in some applications it is not always the most suitable quenching medium available. In the case for the substantially packed bed of hydroprocessing catalyst of the present invention, one major process objective would be to minimize random movement and/or prevent ebullation of the catalyst pellets.
U.S. Pat. No. 5,076,908 to Stangeland et al. teaches the injection of quench gas to control the temperature profile of the reaction zones within the reactor. By carefully selecting an appropriate and essentially the same size, shape, and density of catalyst, and setting up appropriate process conditions regarding oil feed and hydrogen velocity, viscosity and density, predictable "subsequently packed catalyst bed" conditions can be achieved and assured in the lower reaction zone of an upflow substantially packed bed reactor with associated countercurrent plug flow (also referred to as piston flow) catalyst movement. Those skilled in the art of design and/or operational behavior of upflow-type reactor systems, of either packed bed type or ebullated bed type, will recognize that the three major influences to catalyst movement behavior within a hydroprocessing reactor are as follows: (1) catalyst pellet size, shape, and density; (2) liquid velocity, viscosity, and density; and (3) gas velocity and density.
One factor which disturbs process operating balances within a hydroprocessing reactor and causes gas to "channel" the catalyst, or causes catalyst to be ebullated into random motion, is gas velocity increase in reaction zones above each gas quench injection point within the catalyst bed of a hydroprocessing reactor. A hydroprocessing reactor containing four reaction zones, for example, would probably have three quench gas injection points or levels. If the hydroprocessing process is set up to accomplish substantial hydrogen upgrading, exothermal heat rise across each reaction zone will dictate that substantial quench gas flows or quantities be injected in excess of any hydrogen gas being consumed by the hydroprocessing process in order to control the desired reactor temperature profile. As the quench gas injected combines with the original hydrogen feed gas and any gas produced by hydrogenation reactions within the catalyst bed of the reactor, the gas superficial velocity increases in each succeeding reaction zone approaching the top of the hydroprocessing reactor. Stated alternatively, the velocity of gas flowing upwardly through the hydroprocessing reactor and along the axial length of the catalyst bed accelerates as quench gas is injected into the catalyst bed at desired levels therein to satisfy cooling requirements for reactor temperature profile control.
When fresh catalyst is first loaded into the top reaction zone of an upflow packed bed reactor (with countercurrent catalyst movement) such as that disclosed in U.S. Pat. No. 5,076,908 to Stangeland et al, the catalyst is at its lowest density level and most vulnerable to radical or unpredictable movement at higher gas velocities. The catalyst in the lower reaction zones are spent catalyst with a higher density level and possesses a more predictable catalytic movement. For example, assuming all other process variables are more or less the same except for gas velocity, appropriately sized and shaped spent catalyst pellets at their highest density at the bottom of a hydroprocessing reactor might be able to tolerate superficial gas velocities in the range of about 0.16 foot per second and still remain in a substantially packed, non-ebullated condition. This same catalyst pellets in their freshest, least dense condition at the top of the reactor might only be able to tolerate gas superficial velocity in the range of about 0.11 foot per second without being ebullated or otherwise expanded into random motion or experiencing "channeling".
In an upflow packed bed type reactor, ebullation or expansion of the catalyst into random motion is known by those skilled in the art to cause higher than desirable attrition rates of the catalyst and could adversely affect distribution. "Channeling" is an undesirable phenomenon that creates serious maldistribution problems, resulting in localized hot or cold zones, effecting reactor performance and, in extreme cases, reactor safety. Extremely high catalyst attrition rates are also experienced when conditions of catalyst bed channeling exist in an upflow hydroprocessing reactor. Continuing to assume that the previously stated above superficial gas velocity tolerable limits of about 0.16 foot per second for spent catalyst at the bottom of a hydroprocessing reactor and about 0.11 foot per second for fresh catalyst at the top of the reactor are viable for exemplary purposes only, the design criteria of an upflow substantially packed catalyst bed hydroprocessing reactor accomplishing substantial hydrogen upgrading, must consider the influence of upwardly pyramiding (or accelerating) superficial gas velocity through the reaction zones and along the axial length of the catalyst bed. For example, if the design basis hydrogen feed rate is set to maintain superficial gas velocity of about 0.10 foot per second at the inlet to the first reaction zone in the bottom of the hydroprocessing reactor, the accumulative effect of quench gas, manufactured gas, and vaporization can increase the superficial gas velocity in the final reaction zone in the top of the hydroprocessing reactor to a velocity in excess of tolerable limits and "substantially packed bed" conditions will be substantially disturbed. If protective counteractions are taken by reducing the design basis hydrogen feed rate into the first reaction zone, capacity and economics of design would be compromised.
A more acceptable design protective counteraction, and a method normally selected, is to reduce the quench gas flow as low as possible and operate with a maximum tolerable temperature rise between the feed inlet to the first reaction zone and the temperature of the effluent from the final reaction zone. While this type of design protective counteraction is a viable solution, there is an economical price to be paid in terms of catalyst replacement rate.
It is a known phenomenon to those skilled in the art that fresh hydrogenation catalyst will experience abnormally high carbon deposition fouling rates during its early life in a hydroprocessing reactor. The rate of fouling during this period of high activity level is greatly influenced by temperatures that the catalyst is subjected to and the period of time the catalyst is in residence in high temperature zones. By lowering the temperature in reaction zones where fresh highly active catalyst is in residence, the useful life of the catalyst will be enhanced or extended. Therefore, while minimizing quench gas injection and operating with a steep reactor temperature profile is considered to be a viable solution, it is not the optimum solution regarding catalyst usage and life because of the high temperature exposure to the fresh catalyst.
It has now been discovered that a major potential bottleneck to economical design can be overcome, operability enhanced, and catalyst attrition (a direct influence to catalyst life) minimized by shifting part of, or in some cases all of the interstage quench fluid cooling load from recycle gas to process liquid that has been recovered and preferably cooled. By selecting quench liquids not subject to excessive vaporization at reactor conditions, overall gas velocity upwards through the reactor can be substantially reduced. Instead of a gas profile of increasing gas velocity through the upper reaction zones of a substantially packed bed reactor, the gas velocity can be regulated within a narrow range which would be more compatible to maintaining "substantially packed bed" conditions by maintaining the appropriate balance of gas and liquid quench flows. In other words, starting with an assumed gas superficial velocity of about 0.10 foot per second into the first reaction zone, the hydrogen chemically consumed might reduce the gas superficial velocity at the outlet of the final reaction zone to about 0.08 foot per second, were it not influenced or replenished by gas produced within the reactor vessel by the hydrogenating process, and additional recycle hydrogen injected for interstage quenching. Depending on the severity of the application and other process condition factors, the gas produced within the reactor might replenish about 0.015 to 0.025 foot per second superficial gas velocity, or more or less maintain the original gas velocity of about 0.10 foot per second through all reaction zones. Total quench gas requirements for "heat sink" to remove the heat of reaction and control the desired reactor temperature profile, might add about 0.02 foot per second, increasing the superficial gas velocity in the final reaction zone to about 0.12 foot per second. If only enough gas quench is injected into the catalyst bed to maintain optimum hydrogen partial pressure conditions, the overall superficial gas velocity could be regulated to less than about 0.107 foot per second. The balance of quench fluid required for temperature control would be from a liquid quench supply. Because the quenching liquid selected should not be subject to significant vaporization at reactor conditions, the ability to control the reactor temperature profile at optimum conditions regarding catalyst fouling rates is not restricted by gas velocity criterion. By operating at higher temperatures in the lower sections of an upflow packed bed reactor, more work that creates exothermic temperature rise is accomplished in these lower sections with catalyst suitably aged or conditioned so as not to experience high carbon deposition rates as that for fresh catalyst. A "flatter reactor temperature profile" can be achieved that will extend catalyst life.
Those skilled in the art of hydroprocessing in upflow type reactors, will also recognize the benefits of reducing superficial gas velocity as it relates to bubble size and gas holdup, yielding an increased liquid residence time and lower liquid space velocity. Oil penetration to the internal surface area of catalyst pellets will be enhance by the overall decrease in oil viscosity in the upflow reaction zones, influenced by the lighter, less viscous quench liquid. The lighter, less viscous liquid will also affect fluidization coefficients in a favorable manner, minimizing the potential of ebullating the catalyst into random motion, thereby minimizing the attrition rate. In the case of two or more upflow packed-bed type reactors in series performing substantial hydrogen upgrading, the benefits of minimizing gas velocity through the reaction zones becomes more pronounced.
The prior art does not teach or suggest any of the particular methods and/or systems of the present invention for quenching in hydroprocessing of a hydrocarbon feed stream, and for extending a life of hydroprocessing catalyst in a hydroconversion reactor, especially a life of a substantially packed bed of hydroprocessing catalyst which is plug-flowing downwardly (or otherwise moving downwardly) in a hydroconversion reaction zone during hydroprocessing by contacting the hydroprocessing catalyst in the hydroconversion reaction zone with an upflowing hydrocarbon feed stream having a liquid component and a hydrogen-containing gas component.
Principal objectives and direction of development of catalyst for hydrocarbon processing ebullated bed reactors have been toward very small high surface area catalyst that can be easily expanded (i.e. lifted into random motion) within the reactors with minimum recycle oil requirements. Basic teachings and practice of technologies implementing expanded or ebullated bed reactors, emphasize achieving a "backmixed" reaction zone by the motion of the various size, shape and density catalyst, to promote isothermal temperature conditions. The following prior art patents regarding quenching, especially liquid quenching, as related to hydrogen upgrading in general confirms the unique and novel features and advantages of the present invention:
U.S. Pat. No. 3,425,810 to Scott discloses various schemes for introducing or withdrawing different feed liquids between reaction zones internal to a fixed bed reactor for upgrading consideration.
U.S. Pat. No. 3,489,674 to Borst Jr. discloses a method for hydrodesulfurizing hydrocarbons preferably boiling up to about 1100.degree. F. by subjecting feed hydrocarbons to reaction with hydrogen over hydrogenation catalyst so that the feed hydrocarbons are at least mildly hydrocracked and substantially desulfurized. The reactor effluent is quenched with a specific liquid hydrocarbon stream which had been previously separated from the reaction zone effluent product. The amount of quench is responsive to the measurement of the temperature of the vapor stream out of the high pressure separator immediately following the reaction zone such that a predetermined temperature thereof (below about 800.degree. F.) is maintained in this vapor stream. Hydrocarbon products of reduced content are subsequently recovered.
U.S. Pat. No. 3,728,249 to Antezana et al discloses a scheme to inject various feed oils between reaction zones of a fixed bed reactor as a quenching fluid with the combined objective of introducing additional feed on a selective basis within the reactor.
U.S. Pat. No. 3,841,981 to Layng discloses a process for refining a tar sand derived material selected from the group consisting of a natural tar and bitumen. The refining process includes the steps of passing the feed substantially in the liquid phase through a reaction zone in the presence of particulate contact material and a hydrogen containing gas under temperature in the range of 700.degree. to 850.degree. F. and hydrogen partial pressure in the range of 400 to 2000 p.s.i. wherein the contact material is maintained in an ebullated state by the passage of fluids through the reaction zone and an effluent is removed from the reaction zone and passed to a separation zone and wherein the effluent is separated into at least a light oil fraction and a heavy oil fraction. The improvement in the refining process comprises quenching the effluent with a compatible oil fraction to a temperature below coking at a point prior to the separation of the effluent into its fractional components.
U.S. Pat. No. 4,324,642 to Duraiswamy teaches a process for the production of condensed, stabilized hydrocarbons by the flash pyrolysis of carbonaceous materials (e.g. coals, gilsonite, tar sands, oil shale, organic wastes, etc.). The material is subjected to pyrolysis in the presence of a carbonaceous solid heat carrier and a beneficially reactive transport gas for inhibiting the reactivity of the heat carrier and the char product. A produced gaseous pyrolysis product is quenched by a fluid containing a `capping agent` which stabilizes and terminates newly formed volatilized hydrocarbon free radicals. Hydrocarbons of at least four carbon atoms are condensed and the stabilized liquid product is fractionated.
U.S. Pat. No. 4,356,077 to Che discloses a production method of light aromatics, intermediate coal liquids, tar acids and heavy hydrocarbons is effected by the pyrolysis of coal. The characteristic features of the production method comprise (i) effecting the pyrolysis in the presence of a particulate carbonaceous heat carrier and a beneficially reactive gas active to reduce polymerization and cracking of the pyrolysis vapors by inhibiting the reactivity of the particulate solids remaining after the pyrolysis, including char, and (ii) quenching the pyrolysis vapors with a quench liquid comprising an H-donor solvent.
U.S. Pat. No. 4,357,228 to Che also teaches a process for the production of light aromatics, intermediate coal liquids, tar acids and heavy hydrocarbons by the pyrolysis of coal. The characteristic feature of this process comprises (i) quenching the pyrolysis vapors with a quench fluid comprising an H-donor solvent to form a liquid mixture containing a condensate, (ii) vacuum flashing the mixture to recover various components including a condensate remainder, (iii) hydrogenating the mixture by heating to transfer H from the quench liquid to the condensate remainder, and (iv) separating the hydrogenated mixture into various components. The quenching method used eliminates secondary reactions in the pyrolysis products and hydrogenates the pyrolysis products using relatively mild operating conditions to economically enhance the yield of low-molecule wt. liquids from the process.
U.S. Pat. No. 4,446,003 to Burton et al discloses recovery of heat from the gas leaving a unit for thermal hydrogenation of an oil feedstock is effected by (a) quenching the gas with a liquid in a quench zone in which all the thermal surfaces are irrigated by the quench liquid, and (b) passing the cooled gas and quench liquid to a heat-recovery unit in which all surfaces in contact with the gas are irrigated by the quench liquid. The process prevents fouling of the heat recovery system by heavy aromatic compounds.
U.S. Pat. No. 4,536,278 to Tatterson teaches a process for producing shale oil comprising the steps of: (a) feeding raw oil shale to a retort including a screw conveyor retort with a surge bin, a rotating pyrolysis drum with an accumulator, a rotating trommel screen, a fluid bed retort, a static mixer retort with a surge bin, and a gravity flow retort; (b) feeding combusted oil shale at 1000.degree.-1400.degree. F. into the retort; (c) retorting to produce a dust laden effluent comprising hydrocarbons and particles of 1-1000 microns in size; (d) withdrawing the product stream; and (e) enhancing dedusting of the stream by injecting a normally liquid hydrogen donor quench to stabilize and limit polymerization, and enhance agglomeration. Shale oil is produced and stabilized effectively and efficiently to retard polymerization, aging and decreptitation.
U.S. Pat. No. 4,832,831 assigned to Carbon Fuels Corp. discloses refining coal to produce a slate of hydrocarbon containing co-products by short residence time hydrodisproportionation having a thermal efficiency greater than 75%.
The refining process comprises contacting the coal at a volatilization temperature of 900.degree.-1600.degree. F. and 100-1200 psig for 0.2-2 seconds, with a hydrogen-donor rich atmosphere to yield char and hydrocarbon-containing vapor which is cooled by direct quench with a recycle heavy oil stream to a temperature 100.degree. F. less than the volatilization temperature and to a final temperature of 850.degree. F. with recycle water and oil. Oil-type transportable fuel systems are produced without use of hydrogen from external source.
None of the foregoing prior art patents teach or suggest the present invention wherein a process is broadly provided for: forming a plurality of annular mixture zones under a hydroconversion reaction zone having a substantially packed bed of hydroprocessing catalyst such that each of the annular mixture zones contains a hydrocarbon feed stream having a liquid component and a hydrogen-containing gas component and wherein the annular mixture zones are concentric with respect to each other and are coaxial with respect to the hydroconversion reaction zone; introducing the hydrocarbon feed stream from each of the annular mixture zones into the substantially packed bed of hydroprocessing catalyst to commence upflowing of the hydrocarbon feed stream from each of the annular mixture zones through the substantially packed bed of hydroprocessing catalyst; injecting a flow of a quenching matter (i.e. quenching liquid and/or gas) into the substantially packed bed of hydroprocessing catalyst; withdrawing a volume of particulate catalyst from the hydroconversion reaction zone to commence essentially plug-flowing downwardly the substantially packed bed of hydroprocessing catalyst within the hydroconversion reaction zone; and adding a volume of catalyst to the hydroconversion reaction zone to replace the withdrawn volume of particulate catalyst. None of the foregoing prior art patents teach or suggest the particular present invention which broadly comprises a quench system or assembly which is intended to be utilized in hydroprocessing a hydrocarbon feed stream that is flowing through a catalytic bed in a hydroconversion reaction zone.